Polycarbonates are amorphous thermoplastic materials conventionally obtained by polycondensation of diols and diphenyl carbonate, phosgene or diphosgene.
The toxicity of the phosgene, of the diphosgene or of the phenol inevitably formed in the event of the use of diphenyl carbonate constitutes a major disadvantage in the synthesis of polycarbonates.
The development of polymeric materials resulting from biological resources renewable in the short term has become an ecological and economic imperative in the face of the exhaustion and of the rise in the cost of fossil resources, such as oil.
In this context, the use of dianhydrohexitols, resulting from plant (poly)saccharides, as dihydroxylated monomers in polycondensation reactions appears to be a promising approach to replacing monomers of petrochemical origin.
The preparation of isosorbide-based polycarbonates has been described in patent application EP 2 033 981. This document describes the polycondensation of a mixture of isosorbide, of at least one second alicyclic diol and of diphenyl carbonate. The process exhibited a disadvantage, already mentioned above, of generating phenol as byproduct of the polymerization reaction.
The paper by Saber Chatti, entitled “Cyclic and Noncyclic Polycarbonates of Isosorbide (1,4:3,6-dianhydro-D-glucitol)”, in Macromolecules, 2006, 9061-9070, envisages various routes for the synthesis of isosorbide-based polycondensates. A first synthetic route, consisting in heating isosorbide in the presence of four molar equivalents of dimethyl or diethyl carbonate and in the presence of a catalyst chosen from potassium tert-butoxide (KOtBu), tin dioctanoate (SnOct2) and titanium tetrabutoxide (Ti(OBu)4), at temperatures of between 100° C. and 200° C., is described as not making it possible to obtain isosorbide polycarbonates. According to the authors of this paper, unreacted isosorbide is recovered after reacting at 200° C. for more than two hours. This failure was confirmed by the Applicant Company, which observed that heating a mixture of isosorbide and of dimethyl carbonate in the presence of potassium tert-butoxide, tin dioctanoate or titanium tetrabutoxide resulted in mixtures comprising a high proportion of isosorbide alkyl ethers and a low proportion, indeed even zero proportion, of isosorbide methyl carbonate. These three catalysts thus prove to also or solely be etherification catalysts and not solely transesterification catalysts, as desired in the present invention. They do not make it possible to selectively form dianhydrohexitol di(alkyl carbonate)s and thus, indirectly, cannot be used for the purpose of the preparation of polymers (polycarbonates).
The paper by Saber Chatti describes two other synthetic routes which, however, both suffer from the disadvantage of requiring the use of toxic, indeed even highly toxic, reactants or solvents (phosgene, diphosgene, pyridine, isosorbide bischloroformate).
The preparation of dianhydrohexitol di(alkyl carbonate)s has also been described in patent application US 2004/241553. This document describes a process for the manufacture of dianhydrohexitol di(alkyl carbonate) by reaction of a dianhydrohexitol and of a chloroformate ester. This manufacturing process exhibits the major disadvantage of involving a toxic compound, i.e., a chloroformate ester.
The document JP 6-261774 also describes a process for the preparation of dianhydrohexitol di(alkyl carbonate) (example 5). However, this preparation process again here involves toxic chloroformic entities.